May 19, 2005

Out of Africa does not explain human genomic diversity

Eswaran, Harpending, and Rogers have written a new article in the Journal of Human Evolution which argues strongly against the simple Out-of-Africa model, according to which modern humans are exclusively derived from a relatively recent (i.e., tens of thousands of years old) population originating in Africa. It is, however, also different from the Multiregional model, according to which humans are descended from Homo erectus populations exchanging genes for a long time, as well as the Mostly out of Africa model, which sees that humans are descended from an expanding African population which hybridized with non-Africans to a limited extent.

The Eswaran et al. model does not postulate any particular process of migration, but rather the spread of a modern "gene complex" which emerged in Africa. This complex, consisting of a few co-adapted genes conferred a selective advantage, and gradually spread via gene flow from its original source. According to the authors, this model explains the contradictory stories of different genetic loci: some loci were part of the selective sweep that originated in Africa, and hence show a pattern of radiation from Africa, whereas other loci show no such pattern and represent the persistence of much older genomic ancestry.

The authors' final paragraph summarizes their perceived importance of the new model:

While each locus has its own history, the above reasoning suggests that African-dominated loci would all roughly tell the same story, while the others would each have its own—for assimilation would have varied in time and place in each case. Thus in the late 1990s, after a decade when most geneticists became convinced of the strict replacement recent African origin model, there was confusion when many nuclear loci—each in its own way—contradicted the patterns first seen in mtDNA. Yet, the following of that particular model remained strong, as there was no other theory that could explain the contrasting patterns. Now there is such a theory, and it tells us that while modern humans first emerged in Africa, living human populations carry within them a substantial genetic inheritance that had its origins in non-African archaics.

Journal of Human Evolution (Article in Press)

Genomics refutes an exclusively African origin of humans

Vinayak Eswaran et al.

Abstract

Ten years ago, evidence from genetics gave strong support to the “recent Africa origin” view of the evolution of modern humans, which posits that Homo sapiens arose as a new species in Africa and subsequently spread, leading to the extinction of other archaic human species. Subsequent data from the nuclear genome not only fail to support this model, they do not support any simple model of human demographic history. In this paper, we study a process in which the modern human phenotype originates in Africa and then advances across the world by local demic diffusion, hybridization, and natural selection. While the multiregional model of human origins posits a number of independent single locus selective sweeps, and the “out of Africa” model posits a sweep of a new species, we study the intermediate case of a phenotypic sweep. Numerical simulations of this process replicate many of the seemingly contradictory features of the genetic data, and suggest that as much as 80% of nuclear loci have assimilated genetic material from non-African archaic humans.

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